High Speed PCB Layout Guidelines: Placement Tips and Strategies

In real-estate, the buzz word is; “location, location, location”. Interestingly enough, the same can be said of high speed PCB design parts placement. Although all aspects ofthe high speed PCB design process are important, the placement is especially so. Placement methods that work without a problem in a standard PCB design may not satisfy the critical signal flow requirements of a high speed design. For the design to work, it really is all about “location, location, location”.

Here are some tips and strategies to consider for high speed designs. First, we will look at some basic placement considerations for high speed designs followed by the benefits of creating a floor-plan of the placement before any actual parts are put on the board. Lastly, but certainly not the least in importance, we will discuss termination resistors and where they should be placed.

High speed PCB design part placement

You should avoid placing sensitive high speed devices close to the edge of the board. This is because the edge of the board has different impedance characteristics, and there is a greater chance of electromagnetic interference (EMI). Additionally, cables use ‘connectors’ to connect to your board, and these can radiate EMI. As a result, you’ll want to place connectors close to the board edges and sensitive high speed devices close to the board center, to reduce EMI on your design.

Thermal effects are another aspect of high speed design placement to consider. This is because high speed devices may run at higher temperatures than standard components. To ensure that your hot components placement stay cool, plan your placement so that these components receive an unrestricted airflow. For example, don’t place taller components, like connectors, in the direction of airflow to a hot BGA.

PCB circuit flow

Preparing for placement by floorplanning your parts

Creating a floor-plan of your parts placement is an effective way to prepare. By planning ahead, you can account for small parts like termination resistors and bypass capacitors. This way they will get placed appropriately instead of being squeezed in at the end of your design. Additionally, a pre-placement floor-plan will also allow you to optimize the placement for signal integrity purposes.

Functional blocks of circuitry such as power conditioning, RF, digital, analog, etc., should be organized and placed as groups in order to minimize signal crossing. A pre-placement floor-plan will allow you to see what the signal flow between functional blocks is and how best to plan for it. For instance, group your power conditioning parts together as much as possible in order that their signals do not have to cross through sensitive areas of RF circuitry. Also, plan your placement to keep connection lengths as short as possible. Additionally, components that are part of high speed signal paths (multiple nets that connect a series of components together), should be placed as close together as possible and not spread apart. You should also consider your routing channels when planning your placement to make sure that you have the necessary space.

When planning out the of placement functional blocks of circuitry, keep in mind the power and ground needs as well. Using continuous power planes is usually preferred, but in the event that the needs of the design necessitate a split power plane for multiple voltages, exercise caution placing connected components across the split. High speed transmission lines should not cross splits in power planes as that will break up thereturn path for those signals. Also, avoid placing other components that are not part of a circuit between components of that circuit. This too will affect the return path for that circuit.

Termination resistors

The final and most specific placement strategy is considering the placement of yourtermination resistors. These resistors are often treated as an afterthought drop-in once the main portions of the circuitry have already been placed. Since these resistors are part of the circuit as a whole, their placement is extremely important for it to function correctly. That is why they need to be placed along with the rest of the components in the high speed circuitry instead of after. Although there are various schemes for placing termination resistors based on the needs of the circuit, the following two examples are the most widely used.

Simple Parallel Termination: This scheme puts one side of a termination resistor on the end of the circuit closest to the receiver while the other side is tied to power or ground. The greater the trace length from the load pin to the resistor, the more the circuit is prone to signal reflection resulting in signal degradation. This is why you should place the resistor as close as possible to the load pin of the receiver to minimize the stub length of the connecting trace.

Series Termination: With this scheme, the resistor is placed inline immediately after the driver pin of the circuit. Since the resistor is an inline part of the circuit, it doesn’t have the same stub length concerns as a parallel termination resistor. However, placing multiple series termination resistors for a large scale device will take up a lot of board space around the device. This will require planning ahead to make sure that adequate space is available without having to rip up and replace parts.

Once you’ve decided on how to organize and place your termination resistors, you can add it to your placement floorplan. With your circuitry paths as well as your power and ground needs accounted for, you can create a well thought-out floor plan that will guide your high speed PCB parts placement.